[JSC] Do not skip the FPR path if either payload or tag GPR is unavailable

[emutavchi]

This is a potential fix for the issue #1592

[Copilot]

Pull request overview

This PR fixes a critical bug in the DFG JIT compiler for 32-bit architectures (JSVALUE32_64) that caused crashes during tail call optimization. The issue occurred when the code incorrectly decided to skip the FPR (floating-point register) loading path if either the payload GPR or tag GPR was available individually, when both registers are actually required for a complete JSValue in the 32-bit representation.

• Fixed the logic to only skip FPR path when both tag and payload GPRs are available, not just one
• Refactored the availability check into a lambda function for code clarity

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.